The present invention relates to techniques for eliminating capacitive inrush current. More specifically, the present invention relates to a filter circuit for eliminating capacitive inrush current occurring in electromagnetic coil control circuits controlling the actuator coil of electromagnetic contactors or relays.
Electromagnetic contactors and relays are used for electrically controlling switching operations in power circuits. Essentially, conventional electromagnetic contactors are assembled from three primary elements: a contact structure for carrying current, an electromagnetic assembly for providing the force to close the contacts of the contact structure, and a frame housing for enclosing the contact and electromagnetic assembly. Typically, the electromagnetic assembly comprises a movable armature, a fixed core, and an actuator coil that controls the forces applied to the movable armature. In order to control the current that flows through the coil, a control circuit (often called “electronic coil control”) is provided inside or outside the contactor.
FIG. 4 illustrates such an electromagnetic assembly 100 in a schematic perspective sectional view. According to this example, the actuator coil 102 is arranged at a fixed ferromagnetic core 104. A movable armature 106 is movable along the direction of arrow 108, thereby actuating at least one movable contact (not shown in the Figures). The position of the armature 106 is controlled by the coil current that is flowing in the actuator coil 102. This coil current is controlled by operating a switch which is part of the electronic coil control.
Based on the physical characteristics of the electromagnetic system, the magnetic forces are high when the fixed core 104 and the movable armature 106 are close together. For different reasons, such as for instance energy efficiency, the forces in a closed position are reduced electronically by reducing the electrical power fed to the coil. Conventional electronic coil controls for contactors for instance reduce the power by reducing the duty cycle of a pulse width modulated (PWM) signal that controls the opening and closing of the switch.
FIG. 5 illustrates a conventional electronic coil control 200 that is connected to an electromagnetic DC coil 102 which may be assembled to form an electromagnetic assembly as shown in FIG. 4. The electromagnetic assembly and the electronic circuitry are separate components from a standpoint of circuit logic: In section 212 of FIG. 5, only electronic parts are located, whereas section 214 symbolizes the electromagnetic domain containing the electromagnetic coil 102.
The electronic coil control 200 comprises a first input terminal A1 that can be connected to a positive direct current (DC) supply voltage, and a second input terminal A2 that can be connected to a negative DC supply voltage.
For connecting and disconnecting the coil 102 to/from the DC power supply, a switch S1 is arranged between the coil 102 and the second input terminal A. The switch S1 may for instance be a transistor. The gate terminal G of the transistor is connected to an electronic control unit 206 for controlling the switching of the transistor. Usually, the electronic control unit 206 outputs a pulse width modulated (PWM) signal forming the control signal for controlling the switch S1. For providing a supply voltage to the electronic control unit 206, a supply voltage regulator 208 is connected between the positive input voltage and the electronic control unit 206.
A freewheeling diode D5 is connected in parallel to the coil 102 in order to provide a freewheeling circuit 204.
In order to protect the electronic coil control 200 against conductive emissions, same is equipped with a filter circuit 210 having a filter capacitor C2. However, when the supply voltage is applied, the charging of the filter capacitor C2 during startup generates a high current peak, which is also referred to as inrush current. The inrush current is symbolized by the current path 202 in FIG. 5.
Such current peaks are unfavorable for the upstream equipment and all components of the coil control 200, including the DC power supply device. The inrush current may cause electromagnetic disturbances, can weld contacts, reduce the number of coils that can simultaneously be energized, and subsequently result in reduced operational life, enhanced maintenance cost, and safety issues.
On the other hand, it is known to provide additional inrush current limiting circuits. However, these known circuits add to the costs and complexity of the electronic coil control.
Hence, there exists a need for a technique of limiting inrush currents into inductive loads, in particular into electromagnetic coils that actuate contactors or relays, which is reliable, does not enhance the complexity of the control circuit, and efficiently suppresses inrush currents.